The invention relates to a method of making a planar lightwave guide with light conducting strips by the precipitation out of materials in the gaseous phase on a substrate of thin, glass-like strata in accordance with a programmed schedule thereby allowing a predetermined refractive index curve to be obtained so that a light-conducting core zone and protective sheathing layers adjacent to said core zone are formed, the said light-conducting strips being subsequently produced from this strata system.
Planar lightwave guides are used in optical communication systems as coupling elements for optical wave conductors. Depending on the chosen arrangement, these coupling elements serve the purpose of signal branching and signal mixing, i.e. they serve as demultiplexes/multiplexer elements.
A known method for making these wave guides is the CVD process in which SiCl.sub.4 of high purity is mixed with a few per cent of TiCl.sub.4 and caused to react with oxygen in an open flame. The glass particles which are produced by flame hydrolysis are deposited on a substrate. During the deposition process, the burner is continuously reciprocated so that several layers are formed. The refractive index is controlled by the TiCl.sub.4 current. Thereafter the substrate with the porous glass layers is heated in order that the individual layers will consolidate (Kawachi et al., Electronics Letters 1983, Vol. 19, No. 15, page 583).
The layer system is then covered with a silicon mask and guide grooves for the accommodation of wave conductors as well as light conducting strips are produced by targeted etching (Yamada et al., Electronics Letters 1984, Vol. 20, No. 8, Page 313).
These known planar wave guides have the disadvantage that the refractive index profile through the deposited layer can be predetermined in only one direction, namely in the direction normal to the substrate. After etching, the light-conducting strip has substantially rectangular cross-sectional configuration and the profile of the light-conducting core is not laterally adapted which gives rise to considerable losses due to dampening. Another drawback resides in that only relatively thick layers can be produced so that finely graded refractive index profile cannot be obtained.
A method is known from European Patent Application No. EP-0052901 whereby coupling elements are made with light-conducting strips which are round in cross-section. To this end, grooves having a semicircular cross sectional configuration are formed mechanically or by etching the substrate glass plate in accordance with a predetermined pattern. In the next step, glass-like layers are precipitated out of the gaseous phase onto the glass plate and in these grooves by application of a CVD process. With increasing layer thickness, increasingly more doping material is deposited jointly with the quartz glass. This is continued until the grooves are completely filled by these layers. The same is applied to another substrate provided with the corresponding mirror-image groove pattern. Then both substrates plates are polished and joined so that the grooves with the glass-like layers coincide. Whilst these strip conductors have a circular cross section with a radially outwardly decreasing refractive index, their manufacture is not without problems.
The production process and particularly the polishing operation are very expensive. The grooves must coincide precisely and neither impurities nor air gaps may remain at the seam or junction between the substrate plates in the region of the light-conducting layers.